President Trump unveiled a new strategy for the U.S. war in Afghanistan on Aug. 21. (Victoria Walker/The Washington Post)

On Monday, President Trump made it sound like America is revving up its war with Afghanistan, by an untold number of troops delivering “overwhelming force” over an untold amount of time.

“Our troops will fight to win,” he said in his first major speech on Afghanistan. “From now on, victory will have a clear definition: attacking our enemies, obliterating ISIS, crushing al-Qaeda, preventing the Taliban from taking over the country and stopping mass terror attacks against Americans before they emerge.”

Except, he’s going to need Congress’s help to do that. And it’s not immediately clear how much of it he will get from them.

Constitutional experts say the president can decide to escalate or de-escalate U.S. troops’ fighting in Afghanistan whenever he wants, since Congress authorized the executive branch to use military force in Afghanistan immediately after the Sept. 11, 2001 attacks.

There’s no such thing as a free war, and Congress is the one that will authorize money for Trump’s to-be-determined troop increase. Though Trump didn’t specify how many troops would join the 8,500 serving in the region, congressional officials say the administration pegged the increase at about 4,000 additional troops.

“If Trump wants to deploy more troops to Afghanistan, someone has to pay for it — and that’s Congress,” said Cornell Law Dean Jens David Ohlin.

Congress has a track record of being skeptical of giving a president an open check on conflict-driven foreign policy. They threatened to pull the plug on funding for the Vietnam War to force President Richard Nixon to draw down. They never gave Obama a new authorization of military force for Syria in 2013, and they didn’t give him any money to transfer prisoners from Guantanamo Bay, forcing him to leave one of his earliest campaign promises unfilled.

It’s not too hard to see lawmakers putting Trump — a new commander in chief who used his Twitter account to spike tensions with a nuclear North Korea — under the microscope before they hand him billions of dollars for Afghanistan for an untold number of troops.

“Congress has a legitimate ability to review every aspect of any commitment to combat forces,” said Anthony Cordesman, a foreign policy expert at the Center for Strategic and International Studies.

That’s not to say Congress unilaterally opposes the broad outlines of what Trump wants to do.

Republican hawks like Sen. John McCain (R-Ariz.) have been waiting for a president who will surge troops, not draw them down like President Obama did. And some military generals have said it’s a good idea not to telegraph a timeline of when the United States will leave Afghanistan, again like Obama did.

But other lawmakers are skeptical about ramping up a war that has been going on for 16 years, killed more than 2,400 American troops and now upward of 60 percent of Americans oppose escalating. Trump didn’t assuage their concerns when he failed to back up his war-drum rhetoric with basic specifics, like how many troops he’d send, when, for how long and in what capacity.

(Philip Bump / The Washington Post)

“When President Trump says there will be no ceiling on the number of troops and no timeline for withdrawal,” said House Minority Leader Nancy Pelosi (D-Calif.) in a statement, “he is declaring an open-ended commitment of American lives with no accountability to the American people.”

Rep. Tom Cole (R-Okla.) is a good test case for where Congress could fall on this. He worked in the building across from the White House on 9/11 and said he’s “more patient than others” on how long this war will last. But that doesn’t mean he’s okay with giving the president the go-ahead to pour unlimited troops into the country.

“Just give me something that’s sustainable and realistic,” he told The Fix on Monday, before being briefed on Trump’s plan, “and that I’m comfortable as a member supporting and being willing to ask young men and women to go to a dangerous place. But you gotta believe somehow there’s something other than just an endless war.”

Congress doesn’t just decide to fund or defund a president’s war. The constitution also gives them the power to declare war. Except, many security and constitutional experts argue that over the decades, Congress has abdicated its constitutional authority to be the branch that declares war in the first place.

Most members of Congress today weren’t even in Congress when they passed a 2001 and 2002 authorization for use of military force in Iraq and Afghanistan that is being used today by Obama — and now Trump — to justify fighting there. They’re not even seriously considering passing a new one, despite pressure from senators like Tim Kaine (D-Va.) and Jeff Flake (R-Ariz.) to write one specifically for the Islamic State.

“We’ve been at war now for nearly 17 years and Congress has never insisted on even a costing of the conflict,” Cordesman said.

There are a lot of reasons Congress has shied away from its duties. Authorizing war is not a politically fun vote to take; just ask Hillary Clinton, who voted to authorize force in Iraq and spent the next eight years explaining her “mistake.”

But if Congress wants a say in this next phase of the war in Afghanistan beyond signing the check for troop levels, they may have to take that vote, sooner rather than later.